Parameters which normally are to be taken into account when designing to disposable medicament delivery devices include measures to render it difficult to reuse the device and measures to protect users from needle sticks, especially after medicament administration.
U.S. Pat. No. 7,662,131 B2 discloses a liquid-injection syringe assembly. The assembly comprises a body forming a reservoir for the liquid, the body being provided with a liquid-injection needle, a plunger mounted to be axially movable in the body between a ready position and an end-of-injection position, and a sheath in which the body is mounted to be axially movable between an active position in which the needle projects from a distal end of the sheath, and a protecting position in which the needle is retracted inside the sheath. The assembly also comprises means for axially securing the plunger and the sheath in a relative position of the plunger and the sheath serving firstly to position the body in its protecting position relative to the sheath and secondly to position the plunger in its end-of-injection position relative to the body. In order to obtain the protecting position the syringe body is biased by means of a spring, which resiliently urges the syringe body towards its protecting position within the sheath when the plunger has reached the end-of-injection position.
A disadvantage with the syringe assembly of U.S. Pat. No. 7,662,131 is that it seems as if the plunger flange may snap into the axial securing means upon assembly of the syringe assembly as the plunger is inserted into the sheath from the distal sheath end and must thus be moved past the axial securing means to obtain the ready position. This interlocking of the plunger flange and the axial securing means pre-medicament administration would render it impossible to administer a medicament. Moreover, assuming that the plunger may be set in the ready position and thus enabling medicament administration, the inwardly protruding retaining abutments may obstruct the path along which the spring is to extend, which may result in only partial spring extension post medicament administration leading to undesired needle exposure.